The two isomers of propyl bromide (n-propyl bromide or “NPB”) and isopropyl bromide or “IPB”) are both commercially useful products. NPB is the more widely used of the two. It is desirable to provide them in as high purities as possible.
Methods for producing propyl bromide (i.e., both of NPB and IPB with the amount of NPB predominating over IPB) are known. One such general method involves hydrobromination of propylene using free-radical catalysis. Commonly-owned copending applications (Cases B1-7282 PCT, filed Apr. 13, 2006 with a priority date of Apr. 18, 2005; and B1-7448, filed Apr. 13, 2006) describe preferred hydrobromination reactions of this type. For example, the process of B1-7448 as applied to propyl bromide production comprises continuously feeding propylene, gaseous hydrogen bromide, and a molecular oxygen-containing gas into a liquid phase reaction medium comprised of aliphatic bromide (preferably propyl bromide corresponding to that being produced) to cause anti-Markovnikov addition of HBr to propylene, the feeds being proportioned and maintained to provide a molar excess of hydrogen bromide relative to propylene in the range of about 1 to about 5 percent, and a molar ratio of molecular oxygen to propylene of less than 0.005, and preferably in the range of about 0.00005:1 to about 0.001:1. The principal product is NPB with small amounts of IPB also being co-formed.
Studies in our laboratories have shown that formation of trace amounts of bromine and peroxy radicals are formed during the hydrobromination reaction. This in turn leads to the formation of small amounts of various other by-products such as 1,2-dibromopropane, acetone, bromoacetone, propionaldehyde, 1,3-dibromoacetone, 1-propanol, and 2-propanol. The formation of acetone and bromoacetone, typically at levels of about 200 ppm, is especially undesirable as bromoacetone even at such low concentrations is a powerful lachrymator. Also when it is desired to recover isopropyl bromide as a co-product, acetone is troublesome as it has a boiling point essentially the same as the boiling point of isopropyl bromide and thus is very difficult to separate these materials from each other.
Conventional distillation procedures can be used to purify propyl bromide products (NPB and IPB) formed by the hydrobromination of propylene. However, such distillations typically require expensive distillation facilities operated under close control of conditions if highly pure NPB, and optionally highly pure IBP, are to be recovered.